In the recovery of oil from oil-bearing reservoirs, it is typically possible to recover only minor portions of the original oil in place by primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental techniques have been developed and used to increase oil recovery. A commonly used secondary technique is waterflooding, which involves injection of water into the oil reservoir. As the water moves through the reservoir, it displaces oil therein to one or more production wells where the oil is recovered.
One problem encountered with waterflooding operations is the relatively poor sweep efficiency of the water, i.e., the water can channel through certain portions of the reservoir as it travels from the injection well(s) to the production well(s), thereby bypassing other portions of the reservoir. Poor sweep efficiency may be due, for example, to differences in the mobility of the water versus that of the oil, and permeability variations within the reservoir, which encourage flow through some portions of the reservoir and not others.
Various enhanced oil recovery techniques have been used to improve sweep efficiency. Aqueous solutions containing high molecular weight, water soluble polymers have been employed to improve sweep efficiency. These media are more viscous than ordinary water or brine, but often undergo molecular weight breakdown, degradation and shearing due to temperature, oxidative stress, and physical force of the wellbore. The degradation leads to reduced viscosity and reduced secondary and tertiary recovery rates of oil from subterranean formations.